Elevators are generally guided in an elevator shaft by guide rails that are affixed to the building structure. The elevator generally includes a sling that is hoisted by cables and a cabin that is mounted within the sling. The elevator cabin is normally isolated from the sling by elastomeric dampers, springs, or a combination of springs and elastomeric dampers.
Typically, an elevator car is guided by guide rails in such a manner that guide elements of guide devices provided in the elevator car come into contact with the guide rails, which are vertically arranged on side walls of a hoistway. However, errors frequently occur in the installation of the guide rails such that they are misaligned, and further deflection is often caused in the guide rail by a load given to the car, and a small level difference and winding may be caused in the guide rail with age. Accordingly, the elevator may be vibrated in the up and down direction (elevating direction) and/or the side to side direction (direction perpendicular to the elevating direction). Guide rails are likely never to be perfectly aligned. The misalignment of the guide rails can additionally be caused, for example, by installation errors, building settlement, or building movement, such as occurs in tall buildings during windy conditions. It is not uncommon to find that the misalignment of the guide rails is caused by all of these factors. Additionally, vertical vibrations caused by such things as torque ripple in the drive system may be transmitted to the sling and therefore to the elevator cabin via the ropes. The characteristics of ropes as string resonators are often such that vertical vibrations quickly manifest themselves as horizontal vibrations that are sensed in the cabin. Aerodynamic buffering may also create vibrations in the elevator cabin.
Misalignment of the guide rails and other factors frequently result in vibration that is felt by passengers. Such vibrations are often uncomfortable and may be anxiety inducing to passengers. In addition to being uncomfortable and a psychological stressor, the vibrations also may have a real effect on the life expectancy of various elevator components due to inconsistent wear and/or consistent or frequent detrimental vibratory stress.
Conventionally, in order to reduce the longitudinal and the lateral vibration, an elastically supporting member or a vibration isolating member for reducing an input of displacement given by the guide rail is arranged between the cage and the car frame or between the car frame and the guide element. In such situations, generally, to provide significant isolation of vibration, it is necessary to reduce the rigidity of the elastically supporting member and the vibration isolating member. On the other hand, in order to prevent the occurrence of interference of the cage with other components when an unbalanced load is given to the cage, it may be necessary to somewhat increase the rigidity. For the above reasons, it may be difficult to design an elevator for which a sufficiently high vibration isolating effect can be provided where, concomitantly, no problems are caused even if an unbalanced load is given to the cabin.
Numerous systems have been developed in attempts to attenuate longitudinal and lateral vibrations. Many of such systems are based on the sky hook dampener concept. U.S. Pat. No. 6,474,449, the disclosure of which is incorporated herein by reference, teaches such a system that uses an approach that produces a constant vibration correcting force regardless of the position of the actuator, the asymmetric load in the car, or the disturbing force. In such systems, attention is generally given to an active vibration isolating method, in which a force to suppress vibration is given from the outside, instead of a passive vibration isolating method such as a damper. In the '449 patent, an active vibration isolating method is disclosed in which an electric current is made to flow in a coil so as to generate a magnetic field at the center (axial center) of the coil. Also, vibration is reduced by a magnetic force when a reaction bar made of magnetic body is arranged at a position opposed to the magnetic field.
In addition to reducing vertical and horizontal vibration, numerous elevator safety systems have been developed to protect passengers and components in the event of a mechanical failure or environmental event. Roller guides are generally equipped with stops that limit their travel. For example, if excessive travel exists, then the braking shoes of the associated safety gear will contact the rails of the elevator and may then engage the brake shoes bringing the cabin to an emergency stop.
In seismic areas, auxiliary guiding means may be provided at each guide shoe to continue to guide the elevator cabin even if the normal guide shoes have failed such as, for example, during an earthquake. However, the auxiliary guide rails are often simply notched steel plates, where the contact between the steel plates and the rails may produce an uncomfortable ride for passengers.
Elevator cabins are normally loaded in such a way that the center of gravity of the cabin does not coincide with the center of suspension. These circumstances may cause the cabin to tilt and also may cause the springs or the roller guide to be compressed unequally. While this condition exists routinely with passive roller guides, it can create special problems for active systems. In order to prevent these conditions, roller guides may be provided with mechanical stops that limit their travel. If a cabin is asymmetrically loaded in an extreme condition an active roller guide may be dictated to move in a direction that will cause impact with one of the stops. Such an impact may be uncomfortable to the elevator passengers and may start or exacerbate an unstable condition in which the active damping system goes into resonance. Such a condition may be anxiety producing, damaging to the elevator system, or dangerous for the passengers.
An actuator described in U.S. Pat. No. 6,474,449 has an almost linear force profile over its displacement range, such as shown in FIG. 1. While such a system may be easy to control under normal operating conditions, it may not prevent or control runaway instability or resonance.
European Patent Application EP-01547955A1 teaches that all closed loop drive systems can become unstable and oscillate to resonance. This is particularly true of elevator active guidance systems. The described system disconnects the active guidance system when it becomes unstable. Although this approach may stop the instability, it may also eliminate the ride quality that an active system attempts to achieve. Additionally, such a system may not be cost effective.
Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings.